Operation of the Telescope Array, TALE, and the TAx4 Expansion by the University of Utah

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The Telescope Array (TA) is the largest detector of ultra-high-energy cosmic rays (UHECR) in the northern hemisphere. TAx4 is an expansion currently under way which will double the number of surface detectors (SD), and since SD deployment can occur along with operations, by 2025 TA will have tripled the existing data set. This award supports the cosmic ray physics group at Utah for one year of running the Telescope Array (TA) complex, including the Low Energy Extension (TALE) and TAx4. This covers managerial, engineering and technical staff effort and expenses. This constitutes a somewhat lower level of support compared to past years, which will reduce the number of nights of collecting data, and provide for a smaller amount of site and equipment maintenance. The Japanese and Korean partners will concentrate on construction and deployment of the remaining SD units, and maintenance of the new SD wings. The Utah staff have sole responsibility for facility maintenance, staffing observing shifts and handling data, and maintaining compliance with Bureau of Land Management land use regulations. TA supports auxiliary and test experiments, recently hosting a lightning mapping array. The ASPIRE project is the TA’s public education and outreach arm, receiving millions of online visits each year. Direct outreach also reaches tens of thousands of teachers and students.Recent exciting results from TA clearly demonstrate the need for more data from an enlarged experiment. One result is the apparent existence of a “hotspot” with possibly curious properties, such as a different, and apparently position-dependent, cosmic ray (CR) spectrum. Another is the apparent discrepancy between spectra and CR composition at the highest energies as measured by TA and by the Pierre Auger Observatory, the largest UHECR detector but in the southern hemisphere. In the longer term, tripling the amount of TA data will help to clarify these comparisons, and continuing TA operations is therefore very important.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.